We consider uncertainty in relation to clinical trials for terminal non-small cell lung cancer, which is an aggressive and difficult to treat form of cancer. Using grounded theory to analyse 85 clinical interactions between doctors, patients and family members, we argue that uncertainty is a major source of tension for terminally ill patients, with individuals confronting a choice between transitioning to palliative care or volunteering for an experimental/trial medication that might postpone death. Regardless of their efficacy, patients must also consider how such experimental treatments might impact their quality-of-life. We argue that clinical trials produce uncertainty through (i) discussions about the efficacy of clinical trials; (ii) the physiological consequences of clinical trial medications; and (iii) the impact clinical trials have on patient's prognostic understanding of their terminal cancer. Accordingly, while study participants encounter high prognostic certainty (i.e. they have a fatal cancer), they nonetheless experience considerable uncertainty in relation to their participation in clinical trials.